Electronic equipment often employs printed circuit boards or cards. These cards typically are mounted in a chassis or card rack by stacking the cards in row alignment with one another. Row alignment in the chassis is defined by slotted or spaced surfaces within or on the chassis, with each card placed in a slot or between a pair of spaced surfaces. Many applications for such retainers require high performance that will captivate a printed circuit board under the most extreme shock and vibration conditions such as those encountered by spacecraft or military aircraft.
Typical printed circuit board retainers are described in U.S. Pat. Nos. 4,823,951 and 5,036,428, the teachings of which are incorporated herein by reference. Such retainers comprise a partially threaded screw or rod and a plurality of members slideably mounted in an end-to-end relationship on the rod. The members disposed on the rod have wedge-shaped end portions which are engagable with one another. The wedge-shaped end portions serve to move at least one of the members in a transverse direction relative to the rod when the members are moved towards one another along the rod. Such movement may be achieved by providing the rod with screw threads to engage and move the distal-most member when the rod is rotated, or alternatively, the movement may result from the action of a lever assembly used-to draw the rod away from the distal-most member, thereby pulling that member toward its companions.
The transverse direction in which the member is moved acts to engage a clamping surface of that member against an edge of the printed circuit board. The opposite edge of the board is thereby forced into contact with, and clamped against, a spaced surface fixedly connected to or integral with the chassis in which the board is to be mounted.
Commonly, the rod in prior art retainers is threaded at one end to engage with mating threads on a nut attached to one of the end-most sliding members mounted on the rod. The opposite end of the rod includes a portion that may be engaged by a tool to allow the rod to be rotated. In so doing, the members are moved toward one another as the threaded rod is rotated in the tightening direction.
In many high performance applications, the printed circuit board chassis comprises a sealed box which does not allow cooling air to pass over the circuit board. Rather, heat generated by the circuit board is conducted through a metal heat sink attached to the printed circuit board and then transmitted to a heat exchanger or plenum. One critical thermal interface in the heat transfer path becomes the printed circuit board retainer itself.
In addition, it is desirable to reduce as much weight as possible in components used in high performance applications such as spacecraft and military aircraft. Typically, weight reduction is achieved by substituting light-weight, and other exotic, materials for conventional materials. For example, aluminum, titanium and various composite materials are used in aircraft to replace materials such as steel.
In the case of the circuit board retainers described above, however, stainless steel has been the material of choice for forming the rod element, due to the perception that lighter materials lacked the tensile strength and other physical properties needed to adequately clamp a circuit board into position. Thus, these retainers continue to use stainless steel rods despite the weight of that material which may cause such rods to account for approximately one-third to one-half of the weight of the entire retainer.
As a result of the use of circuit board retainers in various high performance applications, a need exists for retainers which efficiently transmit or release forces applied between the rod and the slideable members mounted on the rod while minimizing weight of the retainer unit and maximizing its thermal conductance properties.